FIG. 1 is a circuit diagram showing a switching power source apparatus according to a related art (Non-Patent Document 1 and Non-Patent Document 2). In the switching power source apparatus of FIG. 1, a DC power source Vdc1 is connected through a primary winding 5a (the number of turns of n1) of a transformer T to a main switch Q1 that is made of, for example, a MOSFET (hereinafter referred to as FET). Each end of the primary winding 5a is connected to a parallel circuit composed of a resistor R2 and a capacitor C2 and a diode D3 that is connected in series with the parallel circuit. The main switch Q1 is turned on/off by PWM control of a control circuit 100.
The primary winding 5a and a secondary winding 5b of the transformer T are wound to generate in-phase voltages. The secondary winding 5b (the number of turns of n2) is connected to a rectifying-smoothing circuit composed of diodes D1 and D2, a reactor L1, and a capacitor C4. The rectifying-smoothing circuit rectifies and smoothes a voltage (ON/OFF-controlled pulse voltage) induced by the secondary winding 5b of the transformer T and outputs a DC voltage to a load RL.
The control circuit 100 has an operational amplifier and a photocoupler (not shown). The operational amplifier compares an output voltage of the load RL with a reference voltage, and if the output voltage of the load RL is equal to or larger than the reference voltage, narrows the ON width of a pulse to be applied to the main switch Q1. Namely, narrowing the ON width of a pulse to the main switch Q1 when the output voltage of the load RL becomes equal to or larger than the reference voltage controls the output voltage to a constant voltage.
Next, operation of the switching power source apparatus having the above-mentioned configuration will be explained with reference to a timing chart shown in FIG. 2. In FIG. 2, there are shown a terminal voltage Q1v of the main switch Q1, a current Q1i passing to the main switch Q1, and a Q1-control signal to conduct ON/OFF control of the main switch Q1.
At time t31, the main switch Q1 turns on in response to the Q1-control signal, and the DC power source Vdc1 passes the current Q1i through the primary winding 5a of the transformer T to the main switch Q1. This current linearly increases as time passes up to time t32. Like the current Q1i, a current n1i passing through the primary winding 5a linearly increases as time passes up to time t32.
Between time t31 and time t32, the main switch Q1 side of the primary winding 5a is a negative (−) side and the primary winding 5a and secondary winding 5b are in-phase. Accordingly, the anode side of the diode D1 becomes a positive (+) side to pass a current in order of 5b, D1, L1, C4, and 5b. 
Next, at time t32, the main switch Q1 changes from ON state to OFF state according to the Q1-control signal. At this time, excitation energy of the primary winding 5a of the transformer T and energy of a leakage inductance Lg (an inductance not coupled with the secondary winding 5b) are not transferred to the secondary winding 5b, and therefore, are accumulated through the diode D3 in the capacitor C2.
Between time t32 and time t33, the main switch Q1 is OFF, and therefore, the current Q1i and the current n1i passing through the primary winding 5a become zero. Between time t32 and time t33, a current passes in order of L1, C4, D2, and L1, to supply power to the load RL.
According to this switching power source apparatus, insertion of the snubber circuit (C2, R2) relaxes a temporal change of the voltage of the main switch Q1 to reduce switching noise and suppresses a surge voltage applied from the leakage inductance Lg of the transformer T to the main switch Q1.
Non-Patent Document 1: Kousuke Harada “Switching Power Source Handbook,” Nikkan Kogyo Shinbunsha Shuppan, Chapter 2 Basic Circuit and Designing Practice of Switching Power Source, p. 27, FIG. 2.2
Non-Patent Document 2: Kazuo Shimizu “High-Speed Switching Regulator,” Sougou Denshi Shuppansha, 2.2.1 Separately Excited Converter, p. 30, FIG. 2.5